This article will show you how to rename / edit a MergeField in Word. Typing over a MergeField will not update it so it is important you follow the steps below when you want to alter a MergeField.
If you need help inserting MergeFields into Word see Merge DET fields.
In your Word document, right-click the MergeField you want to modify.
Click Edit Field...
Under Field name rename the field to the desired field name.
Click OK. Success! Your MergeField has successfully been modified!